Nowadays natural rubber (one example of polyisoprenoids) for use in industrial rubber products can be harvested from rubber-producing plants, such as Hevea brasiliensis (Para rubber tree) belonging to the family Euphorbiaceae, or Ficus elastica (Indian rubber tree) belonging to the family Moraceae.
At present, Hevea brasiliensis is practically the only one source of natural rubber for industrial rubber products. Hevea brasiliensis is a plant that grows only in limited areas such as in Southeast Asia and South America. Moreover, Hevea brasiliensis requires about seven years from the time it is planted until it matures enough to allow rubber extraction, and the period during which natural rubber can be extracted is limited to 20 to 30 years. Although more natural rubber is expected to be needed, in particular, by developing countries in years to come, for the reason mentioned above it is difficult to greatly increase the production of natural rubber using Hevea brasiliensis. Depletion of natural rubber sources is therefore of concern and there are needs for stable natural rubber sources other than mature Hevea brasiliensis and for improvement in productivity of natural rubber from Hevea brasiliensis. 
For example, an approach to improve productivity of natural rubber from Hevea brasiliensis is to extract more latex to produce more natural rubber. Specifically, such methods include stimulating the trunk of rubber trees with ethylene or ethephon (2-chloroethylphosphonic acid); and accelerating laticifer differentiation using lanolin containing jasmonic acid, linolenic acid (a precursor of jasmonic acid) or the like (see, for example, Non Patent Literature 1).
Unfortunately, if the method of increasing latex production via ethylene stimulation is applied to the trunk for a long term, then cracks may easily be generated in the bark thereof. In addition, the aim of the ethylene stimulation is to allow latex to exude more smoothly from laticifers and is not to directly improve the tree's ability to produce latex, and this method provides only a limited and insufficient increase in latex production.
Although jasmonic acid or the like can be used to accelerate laticifer differentiation and thereby increase the number of laticifers, this method also has the problem that latex exuding from laticifiers can coagulate at the incision during the collection of latex by tapping, and therefore the produced latex may not be sufficiently collected.
Also known are attempts to promote biosynthesis of isoprenoid compounds in plants, such as by overexpressing a gene involved in the mevalonic acid (MVA) pathway or MEP pathway, which are pathways to isopentenyl diphosphate (IPP) biosynthesis, or a gene downstream in such a pathway (Non Patent Literatures 2 and 3).
These methods, however, only enhance the expression of specific enzymes involved in the above-mentioned pathways, or in other words, partially enhances the polyisoprenoid biosynthesis pathway, rather than enhancing the overall pathway of polyisoprenoid biosynthesis. Thus, there remains room for improvement in terms of enhancing the overall pathway of polyisoprenoid biosynthesis.
It is also known that some factors, including light responses, wound responses, and cold treatment, affect polyisoprenoid biosynthesis. However, it is not specifically known which transcription factor is activated in such a response to regulate polyisoprenoid biosynthesis.